This invention relates to a method and apparatus for the evacuation of settled solid materials from the base of field storage tanks used primarily in the oil industry or from vessels employed in chemical downstream processing of heavy crude oil. More specifically, the present invention relates to a de-sanding method and arrangement which utilizes an apparatus for the formation and evacuation of slurries and preferably a storage tank for recovering the contaminated water. The water is re-used in the process and subsequently treated for minimizing consumption and the risk of pollution.
A method for removing fines, such as sand from a crude oil tank is described in U.S. Pat. No. 5,582,652 issued to Robertson et al. The method discloses a process of directing a fluid into the bottom of a tank by means of a coiled pipe to stir up in an uncontrolled manner the fines and create thereby a slurry that can be evacuated from the tank. The second step consists of drawing the slurry through a drain pipe and pumping it into the bed of a truck where the sand is retained for disposal and the liquid is re-circulated.
The method is ineffective in removing the sand from the periphery of the tank due to the fact that the drainpipe is placed transversely across the tank and can reach only a limited area in the vicinity of the pipe. Thus the drainpipe cannot take advantage of the special construction of the inlet line that is designed to distribute uniformly the liquid in the lower region of the tank. Moreover, uniform distribution of the fluidizing liquid cannot be achieved throughout the tank as intended. The diameter of the orifices is the same along the pipe and most of the liquid will tend to exit at one end of the pipe i.e. the central portion of the coil. The supply line orifices are quite small being subjected to intensive erosion that tends to enlarge them and reduce their effectiveness in creating the slurry. At the same time the orifices in the supply line form eddies that tend to emulsify the oil globules circulating through the pipe with the water. The emulsified oil cannot be readily separated within the truck bed contaminating the water employed for slurry creation. This has an undesirable effect on the sand that is recovered from the bottom of the tank and poses a disposal problem both for the water and the sand. The contaminated water that needs to be replaced in order to continue the de-sanding process may be in short supply in certain locations. The slurry also contains fines such as clay that cannot be separated in the bed of the truck because their relative density is very close to the density of water. Consequently the fines in the form of clay also contaminate the water compounding the problem created by the emulsified oil and requiring costly replacement of water.
At the same time no provision is made to isolate the crude oil from the vicinity of the drainpipe and a certain amount of oil is entrained with the slurry resulting in wastage of crude oil and contamination of the water used for slurry formation. The line that supplies liquid for stirring up the fines is prone to clogging and, in order to eliminate the clogging, the de-sanding process must be interrupted with undue downtime.
Prior art methods for removing the fines from crude oil tanks have made no attempt to deposit the fines in a manner that is advantageous for evacuation.
It is therefore an object of an aspect of the present invention to provide an effective method for introducing the crude oil into the holding tank in a fashion that facilitates the sedimentation of the fines mostly in the central region of the tank from where they can be readily removed. This may be achieved in two steps by means of reversing the flow of crude oil in a distributor that absorbs the impact of the fluid entering the tank and then forming a vortex in the lower part of the distributor that is specially designed for this purpose. The distributor is part of a bell shaped apparatus and promotes the flow of liquid in a circular motion that causes the fines to migrate towards the eye of the vortex and settle in the central area of the tank.
It is also an object of an aspect of the invention to provide a method for circulating the fluidizing liquid through the lower portion of the tank without entraining oil in the slurry suction line. The fines are removed by means of a bell shaped device that is lowered onto the surface of the fines. The bell is then flooded with water that displaces the oil from the area under the bell and provides an optimal medium for slurry formation. In addition to isolating the oil from the fluidizing area, the bell also ensures a quiet zone above the de-sanding region preventing the oil from being re-entrained into the bell.
It is also the object of an aspect of the invention to provide an apparatus for fluidizing and removing the sand from the central region of the tank with minimal disturbance and no emulsification. The apparatus comprises an impeller shaped extraction device that causes a swirling motion of the liquid in its vicinity. The swirling motion causes the sand around the apparatus to migrate towards the extraction device. A specially designed inlet at the lower part of the extraction device allows the sand from the area directly below the apparatus to be removed through the suction action of the apparatus. A modified jet pump placed within the central portion of the extraction device forms a vacuum that is uniformly distributed in the lower portion of the de-sanding apparatus. The slurry is then discharged from the tank for processing in a centrifuge.
It is an object of another aspect of the invention to provide a method for removing the fines at a constant rate for preventing turbulence and the overloading of the system. The displacement of the fines causes the slurry extraction device to descend due to gravity and enables it to gradually reach lower areas of the sand sediments. The sand deposit therefore needs agitation only at its surface whereas lower layers remain undisturbed. Consequently the jets employed in removing the upper layer of sand may be relatively weak further minimizing sand disturbance. Meanwhile the sand layers are removed through the downward motion of the de-sanding apparatus that descends due to its own weight. The displacement of an upper layer of sand allows the apparatus to reach the layer immediately below. Due to the fact that the weight of the bell and the flow rate of the fluidizing liquid are constant the slurry concentration also remains relatively constant during the de-sanding process with beneficial effects on the pump and the de-watering device.
It is also an object of an aspect of the invention to provide a method for causing the sand to migrate towards the centre of the tank with negligible agitation of the fluid. This is accomplished by means of distributing uniformly the fluidizing liquid throughout the bottom of the de-sanding apparatus and directing the flow towards the central region of the tank by means of specially designed flow-diverting baffles. The fluidizing liquid has a gentle sweeping action and the sand migrates with minimal agitation and at a constant rate for removal. The de-sanding apparatus tends to form a cavity in the central area of the bell that facilitates the migration of sand from the periphery of the tank.
It is also an object of an aspect of the invention to provide means of fluidizing the fines on the periphery of the tank, the means being resistive to clogging and not causing eddies with inevitable emulsification. The lower portion of de-sanding apparatus has a circular distribution chamber with equally spaced outlet ports. The outlet ports are in fluid communication with nozzle shaped check valves made from elastomers. The duckbill feature of the valve allows the process of free flow in one direction and provides reliable backflow prevention. The nozzles cannot become plugged and undue downtime due to clogs is avoided during the de-sanding process. The nozzle shaped check valve creates a jet that directs the sand towards the centre of the de-sanding apparatus for disposal. The apparatus is placed onto the sand surface slowly by means of a counterweight. This minimizes the impact and prevents the nozzles from being buried in the sand. Consequently the flow through the nozzles remains constant and so does the strength of the jets generated by the nozzles. This ensures an effective sweeping action of the jets without undue disturbance of the sand layers. Moreover, if the nozzles do get buried inadvertently, the pressure exerted by the sand reduces the aperture of the nozzles. Consequently the velocity of the liquid exiting the nozzles is increased and the liquid has an enhanced force for removing the sand from the vicinity of the nozzles. It can be seen that the nozzles are capable of freeing themselves from sand by generating stronger jets when they become buried inadvertently and then readjusting the strength of the jets for optimal performance.
It is also an object of an aspect of the invention to provide an effective method for separating the water from the sand prior to loading the sand into a truck or depositing it for subsequent disposal. A centrifuge well known in prior art may be employed for de-watering the sand and re-circulating the water to the crude oil tank for resuming the de-sanding process. The centrifuge is very effective in separating the water from sand and most of the water returns to the crude oil tank.
It is also an object of an aspect of the invention to provide a method for minimizing the water necessary for the de-sanding process. After fluidizing the sand and being separated from the slurry by the centrifuge, the water is recirculated and undergoes a gravity separation process in the area under the de-sanding apparatus where it releases the oil that may contaminate the fluidizing water. The oil-water separation is assisted by the coalescing effect of the perforated plate of the de-sanding apparatus. The oil then reaches the quiet zone above the de-sanding apparatus and is prevented from being re-entrained in the fluidizing water. The resulting relatively clean water can then be effectively reused for slurry formation.
An object of another aspect of the invention is to provide means for effectively removing the clay from the crude oil tank and water storage tank. The fines that have a relative density very close to that of the fluidizing water cannot be removed during the de-sanding process. However, upon completion of the de-sanding sequence, the water is returned to a water storage tank where it is subjected to gravity separation. Due to sufficient retention time, the fines slowly deposit themselves in the lower region of the tank. The buildup of clay is monitored by means of an optical sensor that initiates a sequence for the automatic removal of clay. Sand that may be transferred from the crude oil tank to the water holding tank is removed in a similar manner when sand accumulation is detected by means of a radioactive sensor.
It is still another object of an aspect of the invention to provide a system formed by a crude oil tank placed in series with another crude oil tank. The system is designed to pass the crude oil through the de-sanding tank for gravity separation of fines. Due to sufficient retention time most of the fines are retained in the crude oil tank for removal through an automatic de-sanding process. The crude oil is then directed to adjacent crude oil tanks for storage. It should be appreciated that there is a negligible amount of fines reaching other tanks in series with the de-sanding tank. The latter do not need a de-sanding system such as the one disclosed above resulting in important savings in terms of equipment costs. The fines that may be carried to other tanks can be removed during the scheduled maintenance of the crude oil tanks.
The present invention contemplates, in a preferred aspect, an electrical control to initiate, monitor and end automatically the de-sanding process in order to ensure the recovery of the fluidizing liquid, the removal of fines from the water storage tank, and automatic replenishing of the water tank.
More particularly, this invention provides an apparatus for removing fines from the bottom inside of a tank, comprising:
an extraction device having
wall means defining an internal chamber,
vacuum means within the chamber which, when activated, lowers the pressure within the chamber,
an inlet device adjacent the bottom of the extraction device, the inlet device being adapted to be adjacent fines accumulated on the bottom of the tank, the inlet device acting, when the vacuum means is activated, to move fines toward the internal chamber by superimposing on the moving fines an inwardly convergent motion, and
operating means for 1) lowering the extraction device toward fines accumulated on the bottom of the tank, and 2) activating the vacuum means.
Further, this invention provides an apparatus for removing fines from the bottom inside of a tank, comprising:
an extraction device adapted to be adjacent fines accumulated on the bottom of the tank, and to remove the fines along a pathway by creating a vacuum effect;
a distributor within the tank and vertically aligned above the extraction device, the distributor having:
a distributor wall means defining an enclosed distributor space,
ducting conduit means for ducting fluid to the enclosed distributor space, said ducting conduit means having at its downstream extremity, within said enclosed distributor space, an outwardly flared portion the effect of which is to decrease the speed with which the fluid enters the enclosed distributor space;
a vortex generating means by which fluid in the distributor is discharged therefrom in a swirling motion to facilitate the separation of fines from the fluid.
Finally, this invention provides a method of removing fines from the bottom inside of a tank, comprising the steps:
providing an extraction device employing the jet pump effect to create a vacuum which, when the device is adjacent the fines, draws the fines into a discharge pathway that directs them out of the tank;
suspending the extraction device above the bottom of the tank,
lowering the extraction device toward and into contact with the fines, and
energizing the extraction device to remove the fines from the tank.